Currently available direct fuel injected two-stroke engines typically use battery-powered inductive spark ignition systems in which spark advance timing is electronically controlled. In such a system, the electronic control module contains an electronic driver for each spark plug which controls operation of the ignition coil providing spark voltage to the respective spark plug. The ignition coil includes a primary winding and a secondary winding. One end of the primary winding is connected to the battery (e.g. 12 volts DC) and the other end of the primary winding is connected to the electronic driver (i.e. a switching mechanism) which is connected to ground. When the electronic control unit instructs the driver to close, electrical current flows through the primary winding and establishes a magnetic field in the ignition coil. To fire the spark plug at a desired time, the electronic control unit instructs the driver to open, thus collapsing the magnetic field inside the ignition coil to induce high tension voltage in the ignition coil secondary winding. The secondary winding is connected to the spark plug, and the high tension voltage creates a spark across the spark plug electrodes to ignite the fuel/air mixture in the combustion chamber. Spark advance timing is typically determined using a look-up table in the electronic control unit based on engine rpm and engine load (e.g. throttle position, manifold air pressure, etc.).
The period of time that the primary winding of the ignition coil charges is referred to as the ignition coil dwell time. Generally speaking, spark energy and spark duration increase with higher ignition coil dwell times. Therefore, higher ignition coil dwell times can be used to reduce cylinder misfires. However, higher ignition coil dwell times can cause premature wear to spark plugs and reduce the life of electronic components.
Two-stroke engines having direct fuel injection tend to have higher ignition energy requirements than conventional two-stroke engines. In a direct fuel injected engine, a stratified charge of fuel is delivered through the combustion head via a fuel injector into the combustion cylinder. The spray is aimed at the spark plug electrodes. At idle and low engine loads and speeds, spark ignition is nearly concurrent with fuel injection. However, even though the fuel spray is aimed at the spark electrodes, the local air/fuel ratio in the vicinity of the spark electrode varies greatly. Therefore, it is important to extend spark duration at idle and light loads to ensure ignition and prevent misfire. Extending spark duration can be accomplished by increasing the ignition coil dwell time.
As engine speed and load increases in direct fuel injected engines, it is necessary to inject more fuel into the cylinder, and it is also necessary to begin injection earlier in time than the occurrence of the spark. At high engine speeds (i.e. near or at the rated engine rpm), strong fluid dynamics within the cylinder fully mix the fuel and air before ignition to create a homogeneous and near stoichiometric air/fuel mixture within the combustion chamber. Under these conditions, ignition occurs easily. However, a combustion process transition region exists between the stratified charge at idle and light loads and the stoichiometric homogeneous charge at high loads and speeds. In the transition region, engine speed is sufficiently high to generate strong in-cylinder fluid dynamics which mix excess air with the injected fuel, thereby diluting the stratified charge. In this combustion region, it is again important that the spark have sufficient energy and duration to ignite overly mixed or lean mixtures. Additionally, in marine applications, fluid dynamics within the cylinder can be unpredictable due to fluctuations in exhaust back-pressure because the engine exhaust is typically routed into the water through the propeller hub. At high speeds (e.g. boat on plane), water motion draws the exhaust through the propeller hub and facilitates effective preparation of the air/fuel charge in the cylinder. However, at the onset of planing, the water can be extremely turbulent in the vicinity of the propeller and can induce a fluctuating exhaust back-pressure which can negatively affect the preparation of the air/fuel mixture within the cylinder. In this situation, it is important to have sufficient spark energy to ignite the mixture.
In order to accommodate ignition energy requirements for direct fuel injected engines at idle, off-idle and in the transition region, current systems maintain ignition coil dwell times at a level sufficient to ensure proper ignition at idle, off-idle and in the transition region. However, as previously mentioned, high spark energy and duration prematurely wears spark plug electrodes, which in turn requires frequent replacement of spark plugs in direct fuel injected engines. High spark energy and duration can also cause electronic component overheating which in turn may reduce the life of the electronic control module.
Another problem with spark plugs in direct fuel injected engines is spark plug fouling. Spark plug fouling occurs primarily at relatively low engine operating temperatures. When spark plug fouling occurs, carbon deposits build up on the spark plug insulator, and the carbon itself becomes a destructive path for the spark so that the spark does not jump the gap between the spark plug electrodes. Spark plug fouling is particularly a problem in direct fuel injected engines because fuel is sprayed directly on the spark plugs.